Gas turbine engines typically include a variety of rotary seal systems to maintain differential working pressures that are critical to engine performance. One common type of seal system includes a rotating member such as a turbine blade positioned in a rub relationship with a static, abradable seal surface. The rub relationship creates a small operating clearance between the turbine blade and seal surface to limit the amount of working gas that bypasses the turbine blade. Too large a clearance can allow undesirable amounts of working gas to escape between the turbine blade and seal surface, reducing engine efficiency. Similar seal systems are typically used as gas turbine engine inner and outer airseals in both the compressor and turbine sections.
To maintain a desirably small operating clearance, the rotating member, for example a turbine blade, typically has an abrasive tip capable of cutting the seal surface with which it is paired. When a gas turbine engine is assembled, there is a small clearance between the rotating member and seal surface. During engine operation, the rotating member grows longer due to centrifugal forces and increased engine temperature and rubs against the seal surface. The rotating member's abrasive tip cuts into the abradable seal surface to form a tight clearance. The intentional contact between the abrasive tip and seal surface, combined with thermal and pressure cycling typical of gas turbine engines, creates a demanding, high wear environment for both the seal surface and abrasive tip.
To limit seal surface erosion and spalling, thereby maintaining a desired clearance between the rotating member and seal surface, seal surfaces are typically made from relatively hard, though abradable, materials. For example, felt metal, plasma sprayed ceramic over a metallic bond coat, plasma sprayed nickel alloy containing boron nitride (BN), or a honeycomb material are commonly seal surface materials.
Unless the rotating member has an appropriate abrasive tip, the seal surface with which is paired can cause significant wear to the rotating member. In addition to degrading engine performance, this is undesirable because rotating members, particularly turbine and compressor blades, can be very expensive to repair or replace. As a result, the materials used to form abrasive tips are typically harder than the seal surfaces with which they are paired. For example, materials such as aluminum oxide (Al.sub.2 O.sub.3), including zirconium oxide (Zr.sub.2 O.sub.3) toughened aluminum oxide; electroplated cubic BN (cBN); tungsten carbide-cobalt (WC--Co); silicon carbide (SiC); silicon nitride (Si.sub.3 N.sub.4), including silicon nitride grits cosprayed with a metal matrix; and plasma-sprayed zirconium oxide stabilized with yttrium oxide (Y.sub.2 O.sub.3 --ZrO.sub.2) have been used for abrasive tips in some applications. Three of the more common abrasive tips are tip caps, sprayed abrasive tips, and electroplated cBN tips.
A tip cap typically comprises a superalloy "boat" filled with an abrasive grit and metal matrix. The abrasive grit may be silicon carbide, silicon nitride, silicon-aluminumoxynitride (SiAlON) and mixtures of these materials. The metal matrix may be a Ni, Co, or Fe base superalloy that includes a reactive metal such as Y, Hf, Ti, Mo, or Mn. The "boat" is bonded to the tip of a rotating member, such as a turbine blade, using transient liquid phase bonding techniques. Tip caps and the transient liquid phase bonding technique are described in commonly assigned U.S. Pat. No. 3,678,570 to Paulonis et al., U.S. Pat. No. 4,038,041 to Duval et al., U.S. Pat. No. 4,122,992 to Duval et al., U.S. Pat. No. 4,152,488 to Schilke et al., U.S. Pat. No. 4,249,913 to Johnson et al., U.S. Pat. No. 4,735,656 to Schaefer et al., and U.S. Pat. No. 4,802,828 to Rutz et al. Although tip caps have been used in many commercial applications, they can be costly and somewhat cumbersome to install onto blade tips.
A sprayed abrasive tip typically comprises aluminum oxide coated silicon carbide or silicon nitride abrasive grits surrounded by a metal matrix that is etched back to expose the grits. Such tips are described in commonly assigned U.S. Pat. No. 4,610,698 to Eaton et al., U.S. Pat. No. 4,152,488 to Schilke et al., U.S. Pat. No. 4,249,913 to Johnson et al., U.S. Pat. No. 4,680,199 to Vontell et al., U.S. Pat. No. 4,468,242 to Pike, U.S. Pat. No. 4,741,973 to Condit et al., and U.S. Pat. No. 4,744,725 to Matarese et al. Sprayed abrasive tips are often paired with plasma sprayed ceramic or metallic coated seals. Although sprayed abrasive tips have been used successfully in many engines, they can be difficult to produce and new engine hardware can show some variation in abrasive grit distribution from tip to tip. Moreover, the durability of sprayed abrasive tips may not be sufficient for some contemplated future uses.
An electroplated cBN abrasive blade tip typically comprises a plurality of cBN grits surrounded by an electroplated metal matrix. The matrix may be nickel, MCrAlY, where M is Fe, Ni, Co, or a mixture of Ni and Co, or another metal or alloy. Cubic boron nitride tips are excellent cutters because cBN is harder than any other grit material except diamond. Electroplated cBN tips are well suited to compressor applications because of the relatively low temperature (i.e., less than about 1500.degree. F. [815.degree. C.]) environment. Similar tips, however, may have limited life in turbine applications because the higher temperature in the turbine section can cause the cBN grits and perhaps even the metal matrix to oxidize. Although electroplated cBN tips are typically less expensive to produce than sprayed abrasive tips, the technology used to make them can be difficult and costly to implement.
Therefore, the industry needs an abrasive tip for gas turbine engine seal systems that is highly abrasive, more durable, and less expensive to produce than those presently available.